The vehicle wheel of the automobile is supported by a rolling bearing unit for vehicle wheel support as shown in FIG. 17 where an example of the conventionally widely used rolling bearing units is illustrated.
This rolling bearing unit 1 for vehicle wheel support comprises a hub 2, an inner ring 3, an outer ring 4 and a plurality of rolling elements 5. The hub 2 has an outer peripheral surface the axially outer end portion of which is formed with a first flange 6 to support the vehicle wheel.
The term "axially outer" means the widthwise outer side when installed in the automobile; the left side in the Figures in this application except for FIGS. 4 to 6, while the term "axially inner" means the widthwise central side when installed in the automobile; the right side in the Figures in this application except for FIGS. 4 to 6.
The intermediate portion of the outer peripheral surface of the hub 2 is formed with a first inner ring raceway 7 while the axially inner end portion of the outer peripheral surface is formed with a step portion 8 with a smaller diameter.
The inner ring 3 is fitted onto the step portion 8 and formed with a second inner ring raceway 9 on its outer peripheral surface.
The hub 2 has an axially inner end portion formed with a male thread portion 10 thereon, the tip end portion of which is projected axially inwards than the axially inner end face of the inner ring 3.
A nut 11 is screwed on the male threaded portion 10. The inner ring 3 is supported between the nut 11 and the step face 12 of the step portion 8, and thereby fixed at a determined position of the hub 2.
The male threaded portion 10 is formed with an engagement recess portion 14 in the outer peripheral surface of its tip end portion. After the nut 11 is clamped with a predetermined torque, a portion of the nut 11 in alignment with the engagement recess portion 14 is crimped radially inwards to prevent the nut 11 from being loosened.
Formed on the inner peripheral surface of the outer ring 4 are a first outer ring raceway 15 which is opposed to the first inner ring raceway 7 and a second outer ring raceway 16 which is opposed to the second inner ring raceway 9. A plurality of rolling elements 5 are provided between the first outer ring raceway 15 and the first inner ring raceway 7 and between the second outer ring raceway 16 and the second inner ring raceway 9, respectively.
Although balls are used for the rolling elements in the figure, tapered rollers may be used for the rolling elements in the case of the rolling bearing unit for the heavy automobiles.
In order to install the rolling bearing unit 1 as mentioned above into the automobile, a second flange 6 is formed on the outer peripheral surface of the outer ring 4, and used to fix the outer ring 4 to a suspension apparatus, thereby fixing the vehicle wheel to the first flange 6. Thus, the vehicle wheel can be rotatably supported by the suspension apparatus.
Disclosed in the specification of U.S. Pat. No. 5,490,732 is a rolling bearing unit 1 for vehicle wheel support, which is illustrated as a second conventional example in FIG. 18.
This bearing unit 1 comprises a hub 18 having an peripheral surface on which a first flange 6 is provided and an inner ring 41 and a second inner ring 3 are fitted. The hub 18 has an axially inner end portion which is projected axially inwards than the axially inner end face of the second inner ring 3 and bent radially outwards to form a crimped portion 19. This crimped portion 19 cooperates with the step face 12 provided on the base portion of the first flange 6 on the outer peripheral surface at the intermediate portion of the hub 18 to support the first inner ring 41 and second inner ring 3 therebetween. Specifically, the axially inner end portion of the hub 18 which is projected axially inwards than the second inner ring 3 is formed with a cylindrical portion which is crimped radially outwards to form the crimped portion 19, which retains the first inner ring 41 and second inner ring 3 onto the step face 12.
In the case of the first conventional example illustrated in FIG. 17, the processes for forming the engagement recess portion 14 on the tip end of the male thread portion 10 and crimping a portion of the nut 11 radially inwards are required. This leads to the cost up due to inconvenient parts production and assembling processes of the bearing unit 1 for vehicle wheel support.
In the case of the second conventional example in FIG. 18, the crimped portion 19 must be formed on the hub 18 to fix the first inner ring 41 and second inner ring 3 to the hub 18. Accordingly, the hub 18 must be made of a material which can be formed with the crimped potion 19. Specifically, in the case of the second conventional example in FIG. 18, since the hub 18 itself is formed with no inner ring raceway, that is since the first and second inner ring raceways 7, 9 are formed on the outer peripheral surface of the first and second inner rings 41, 3 fitted onto the hub 18, the hub 18 can be made of a carbon steel containing less than 0.45 weight % carbon which is easily processed to form the crimped portion 19. However, as the crimped portion 19 is formed, a large load is applied to the second inner ring 3 fitted onto the hub 18, resulting in that the second inner ring 3 is deformed and that the inner clearance (plus or minus) of the rolling bearing unit may be displaced from the required value. When the inner clearance of the rolling bearing unit is displaced from the proper value, the rolling fatigue life of the second inner ring raceway 9 formed on the outer peripheral surface of the second inner ring 3 may be reduced.
Such inconvenience may be produced in the structure where the structure of FIG. 17 and the structure of FIG. 18 are combined to provide the hub 2 with the first flange 6 and the first inner ring raceway 7 and where the inner ring 3 is securely affixed to the hub 2 by way of the crimped portion 19.
In addition, in such a construction, when the hub 18 is made of a carbon steel containing less than 0.45 weight % carbon as in the conventional structure of FIG. 18, it is impossible to make the hardness in the portion of the first inner ring raceway 7 sufficiently large, and to secure the sufficient durability of the portion of the first inner ring raceway 7.